In Vivo Comparison of Tendon Mechanical Properties in Patellofemoral Pain

Introduction: Patellofemoral pain (PFP) is one of the most common abnormalities involving the knee, with reported incidence rates greater than 25% among athletes. Current literature suggests the etiology of PFP is multifactorial, although it is commonly accepted that abnormal patellar tracking is closely related to PFP syndrome. Abnormal patellar tracking may be due to imbalances in the extensor mechanism of the knee. This is the first study to assess the in vivo mechanical properties of the medial vs. lateral tendons of the vastus quadriceps components in both healthy subjects and symptomatic PFP patients. The purpose of this study was to compare the stiffness of the medial and lateral quadriceps tendons in vivo and noninvasively in symptomatic subjects with PFP syndrome to the stiffness of these tendons in healthy subjects. Materials and Methods: The mechanical properties of the tendons from the vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles were examined in 7 healthy volunteers and 7 volunteers with PFP syndrome. All participants provided informed consent and the experimental protocol was approved by the institutional review board. All healthy participants had no current or past symptoms of patellofemoral pain. Experimental procedure: Subjects were asked to perform ramp isometric knee extensions to match target torque levels (3, 5, 7, 9, 15, 20 Nm). Subjects were allowed to practice, with visual and auditory feedback, until they were capable of generating torques within ±1 Nm of the target extension torque while minimizing off-axis torques (<10% of target torque). Practice time served to pre-condition the tendon. A 14MHz ultrasound probe (GE LOGIQ-9, Waukesha, WI) was placed over the distal aspect of the VMO or VL muscle oriented parallel to the direction of the tendon fibers. The elongation of the tendon was recorded continuously and simultaneously with torque. The position of the patella and a proximal point on the tendon were digitized and tendon elongation was calculated for each frame (NIH Image, Bethesda, MD). The tendon moment arms were estimated based on data from the literature and modeling1, and tendon force was calculated. Measurements of tendon length at rest were made using sagittal plane extended field-of-view still-frame ultrasound images of the entire length of each tendon. To measure tendon cross-sectional area, the ultrasound probe was used to record three axial-plane scans at 50% of the length of the tendon. Cross-sectional areas were digitized and tendon stress, stiffness, strain and Young’s modulus (E) were calculated. Considering multiple quadriceps components were activated during voluntary contractions, the VMO/VL stiffness ratio was evaluated in PFP and control groups. Statistical comparisons were made using two-factor ANOVA with post-hoc Bonferroni comparisons. Least-squares linear regression was used and ANCOVA was used to test for significant differences between regression lines; α = 0.05. Results: Table 1. Mean (SD) Subject Characteristics